A variety of medical conditions have been treated by introducing an insertable medical device having a coating for release of a therapeutic agent. For example, various types of medical devices coated with a therapeutic agent, such as stents, have been proposed for localized delivery of such agents to a body lumen. See, e.g., U.S. Pat. No. 6,099,562 to Ding et al issued on Aug. 8, 2000. However, it has been noted that, existing coated medical devices can be improved. For instance, the release profile of a therapeutic agent from the coated medical device may not be accurate or precise in relation to a target tissue site.
In the case of stents, drug-eluting stents are typically stents with a polymeric coating which includes a therapeutic agent. The polymer generally contains a therapeutic agent to, for example, prevent restenosis in a vessel after implantation of a stent. The drug may then be released from a stent into a tissue surface in a controlled manner. Generally, the therapeutic agent is typically only released from the area closest to the outer surface of the coating, leaving a significant amount of therapeutic agent in the coating. Also, current struts generally have rectangular or square outer surfaces. These designs may lead to drug-release at undesirably limited locations thereby not adequately delivering the drug to a target tissue site or vessel wall. This is at least in part due to the fact that at least one surface of a rectangular or square strut is facing away from a tissue surface after a stent has been implanted.
More particularly, FIG. 1 shows a stent strut 5 of a stent that is implanted into a blood vessel 1 having an obstruction 3, such as plaque resulting from restenosis. The strut 5 may have a rectangular shaped cross-section and four sides: an outer surface 5a, which contacts the obstruction 3; an inner surface 5b, which is opposite the outer surface 5a, and two side surfaces or side walls 5c and 5d which connect the outer and inner surfaces 5a and 5b. A coating 7 may be disposed on the four surfaces 5a, 5b, 5c and 5d. As shown in FIG. 1, generally, when the stent with struts having rectangular-shaped cross-sections is implanted, only the outer surface 5a is in intimate contact with the obstruction 3 or blood vessel wall. Therefore, release of the drug or therapeutic agent from the coating is primarily from the outer surface 5a of the stent. Some therapeutic agent can be released from the inner surface 5b and side surfaces 5c and 5d to the obstruction 3 or vessel wall. However, generally the therapeutic agent releases into the blood before reaching the obstruction 3 or blood vessel wall. Thus, it is expected that only a small amount of the therapeutic agent that is released from the inner surface 5b and side surfaces 5c and 5d reaches the obstruction or blood vessel. Also, because therapeutic agents, such as paclitaxel, are lipophillic, the coating generally should be in close contact with the cell walls of the cells that are to be treated for efficient release of the lipophillic agent since cell walls are composed of fat. Since the inner surface 5b and side surfaces 5c and 5d of the coated strut generally do not contact cells to be treated but instead contacts blood, the release of lipophillic drugs from these surfaces tends to be slow.
Inefficient drug release patterns, such as described above, may result in the use of an undesirably high amount of therapeutic agents in a medical device coating to achieve a desired amount of treatment on a target tissue site. Another undesirable result may be the release of unwarranted therapeutic agents directly into the bloodstream. For these and other reasons, there is a need for medical devices, such as stents, with outer surface configurations that more accurately and precisely deliver therapeutic agents to target sites without concurrently releasing undesirably high amounts of such agents away from a target tissue site.
Furthermore, it may be preferable to have a stent with struts that have greater contact with the tissues or cells to be treated. More specifically, it would be desirable to have struts that have coated side surfaces that directly contact the tissue to further more effective release of therapeutic agents from the coated side surfaces to the tissue to be treated. Thus, there exists a need for such a device to more efficiently deliver therapeutic agents to tissue sites.